Nearly one in five gay and bisexual men in 21 major US cities are infected with HIV, and nearly half of them do not know it, US health officials said on Thursday.

Young men, and especially young black men, are least likely to know if they are infected with HIV, according to the US Centres for Disease Control and Prevention.

"For young men who have sex with men – including young men of colour who are least likely to know they may be infected – the future is truly on the line," Dr Jonathan Mermin, director of CDC’s Division of HIVAIDS Prevention, said in a statement.

"It is critical that we reach these young men early in their lives with HIV prevention and testing services and continue to make these vital services available as they become older."

The study tested 8,153 men who have sex with men in 21 cities taking part in the 2008 National HIV Behavioral Surveillance System, which looked at the prevalence of the human immunodeficiency virus and awareness of HIV status among this group of men. HIV is the virus that causes AIDS.

They found that 28% of black men who have sex with men were infected with the HIV virus, compared with 18% of Hispanic men and 16% of white men.

The incidence of HIV in this group of men is strongly affected by education and income, the authors said, noting that the lower a person’s socioeconomic status, they more likely he was to have HIV, CDC researchers reported in the CDC’s weekly report on death and disease.

Age was also a factor in whether men know they are infected with HIV. Among those aged 18 to 29 who had HIV, 63% did not know it, compared with 37% for men aged 30 and older.

That is especially dangerous because the CDC estimates that most new sexually transmitted infections are passed along by people who do not know they are infected.

Mermin said the findings show HIV is still a serious problem in America, and called for a renewed national commitment efforts to protect the health of gay men.

The CDC recommends that gay and bisexual men of all ages get an HIV test each year, and men at highest risk those who have multiple sex partners or use drugs during sex get tested every three to six months.

"We need to increase access to HIV testing so that more MSM (men who have sex with men) know their status, and we all must bring new energy, new approaches, and new champions to the fight against HIV among men who have sex with men," Dr Kevin Fenton, director of CDC’s National Center for HIVAIDS, Viral Hepatitis, STD, and TB Prevention, said in a statement.

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